Fiber optic sensing systems have been developed employing a variety of techniques to provide distributed acoustic sensing (“DAS”) of parameters such as vibration, acoustics, pressure, and temperature, even in hostile environments. Such systems have particular utility in hydrocarbon wells and other downhole environments where space is limited and other types of sensors exhibit sharply reduced reliability. However, fiber optic sensing systems have problems of their own, including interferometric fading and lack of quantitative accuracy.
For example, in a typical DAS system that monitors acoustic activity in multiple channels corresponding to different positions along the optical fiber, at least some of the monitored channels at any given time will exhibit fading due to Rayleigh scattering. All optical fibers have a distribution of impurities that each scatter a small fraction of passing coherent light pulses. The scattered portions of coherent light can interfere constructively or destructively with each other. As the distribution of impurities is perturbed, the degree of interference can be volatile and seemingly random. Where the interference is largely destructive, the signal from that portion of the fiber is suppressed, depleting the signal energy from that channel. The signal-to-noise ratio (SNR) drops, causing large errors and fringe jumping in the phase demodulation process. Typically, the effects are extremely localized, with nearby and even adjacent channels exhibiting no signal energy loss.
This phenomenon is not limited to fiber optic sensing systems. It also occurs in radio, satellite, and wireless communications, and can be particularly severe in heavily built-up urban environments. The prevalence of the issue has led to the construction of statistical models for “Rayleigh fading”. Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission medium (also called a communications channel) vary randomly, fading according to a Rayleigh distribution that is expressible as the radial component of the sum of two uncorrelated Gaussian random variables. Existing DAS systems generally address Rayleigh fading by substantially lengthening the measurement period beyond the expected Rayleigh fade duration or averaging many measurements acquired over such an interval, but the requisite output delays may be unacceptable for many applications. As interference effects vary with wavelength (destructive interference tends to be very localized not only in space, but also in frequency), a proposed alternative approach employs measurements at multiple wavelengths and discarding measurements at those channels and wavelengths that exhibit fading. This alternative undesirably requires additional hardware complexity to collect measurements at multiple wavelengths.